The present invention pertains to improvements in the field of conjugated polymers. More particularly, the invention relates to conjugated poly(2,7-carbazole) derivatives and to a process for the preparation thereof.
A conjugated polymer is a polymer which possesses a delocalized pi-electron system along its backbone as described, for example, by D. J. Sandman in xe2x80x9cTrends in Polymer Sciencexe2x80x9d, Vol. 2, p. 44 (1994). Conjugated polymers are considered as a very important class of electroactive and photoactive materials by both academic and industrial laboratories. The synthesis over the last twenty years of highly pure polyacetylene, polythiophenes, polyphenylenes, polyfluorenes, ladder polymers and other conjugated polymers with optimized physical properties has led to a significant improvement in the performance of these polymeric materials and to a better understanding of their structure-property relationships. However, up to now, only poorly conjugated poly(N-alkyl-3,6-carbazole) derivatives are available so that these are not optimized for the development of light-emitting diodes, electrochromic windows, electrochemical sensors, photovoltaic cells, photoconductors, photorefractive materials, transistors, etc.
It is therefore an object of the present invention to provide conjugated poly(2,7-carbazole) derivatives having improved optical and electrochemical properties.
According to one aspect of the invention, there is provided a conjugated poly(N-alkyl-2,7-carbazole) of formula (I): 
wherein R represents a linear or branched alkyl group having 1 to 22 carbon atoms, and n is an integer of about 3 to about 100.
The present invention also provides, in another aspect thereof, a process for preparing a conjugated poly(N-alkyl-2,7-carbazole) of the formula (I) defined above, which comprises treating a N-alkyl-2,7-difunctionnalized carbazole of formula (II): 
wherein R is as defined above and X represents a trifluoromethanesulfonyl group or a halogen atom selected from the group consisting of bromine, chlorine and iodine atoms, with triphenylphosphine and 2,2xe2x80x2-bipyridine in the presence of zinc and nickel chloride, to cause polymerization of the compound of formula (II). A
According to a further aspect of the invention, there is provided a conjugated polymer comprising alternating or random units of formula (Ixe2x80x2): 
wherein R is as defined above.
The polymers comprising alternating units of formula (Ixe2x80x2) can be obtained by Suzuki on Stille couplings. The polymers comprising random units of formula (Ixe2x80x2), on the other hand, can be obtained by Yamamato-couplings.
The conjugated poly(2,7-carbazole) derivatives according to the invention, comprising repeating, alternating or random units of formula (Ixe2x80x2), have interesting optical and electrochemical properties which render them suitable for use in the manufacture of light-emitting diodes, electrochromic windows, electrochemical sensors, photovoltaic cells, photoconductors, photorefractive materials and the like.